The kinetics of the hydroxylation of hexobarbital and benzo(a)pyrene as well as the conjugation of morphine and oxazepam glucuronic acid have been studied in hepatic microsomes isolated from fetal stump-tailed monkeys (Macaca arctoides) during the last half of gestation. The affinity of the enzyme for each substrate (i.e., apparent Km) remained constant during the time period examined. Different patterns of development were observed for Vmax, depending on the substrate and the reaction.